1. Field of the Invention
The present invention relates to methods for making thin film transistors and, particularly, to a method for making a carbon nanotube based thin film transistor.
2. Discussion of Related Art
A typical thin film transistor (TFT) is made of a substrate, a gate electrode, an insulation layer, a drain electrode, a source electrode, and a semiconducting layer. The thin film transistor performs a switching operation by modulating an amount of carriers accumulated in an interface between the insulation layer and the semiconducting layer from an accumulation state to a depletion state, with applied voltage to the gate electrode, to change an amount of the current passing between the drain electrode and the source electrode. In practical use, a high carrier mobility affect by the material of the semiconducting layer of the thin film transistor is desired.
Usually, the material of the semiconducting layer is amorphous silicone (a-Si), poly-silicone (p-Si), or organic semiconducting material. The carrier mobility of an a-Si TFT is relatively lower than a p-Si TFT. However, the method for making the p-Si TFT is complicated and costly. The organic TFT is flexible but has low carrier mobility.
Carbon nanotubes (CNTs) are a novel carbonaceous material and have received a great deal of interest since the early 1990s. Carbon nanotubes have interesting and potentially useful heat conducting, electrical conducting, and mechanical properties. Further, there are two kinds of carbon nanotubes: metallic carbon nanotubes and semiconducting carbon nanotubes determined by small differences in the diameter and chirality affect. The carrier mobility of semiconducting carbon nanotubes along a length direction thereof can reach about 1000 to 1500 cm2V−1S−1. Thus, TFTs adopting semiconducting carbon nanotubes as a semiconducting layer have been produced.
Methods for making carbon nanotube based TFTs include dispersing an amount of carbon nanotube powder in an organic solvent to form a mixture; printing the mixture on a substrate; volatilizing the organic solvent to achieve a carbon nanotube layer on the substrate; forming a source electrode and a drain electrode on the carbon nanotube layer; forming a silicon nitride layer on the carbon nanotube layer; and forming a gate electrode on the insulating layer.
However, there are problems with the present carbon nanotube based TFTs. Firstly, the carbon nanotubes are prone to aggregate in the mixture. Thus, the carbon nanotubes cannot be uniformly dispersed in the carbon nanotube layer. Secondly, the organic solvent is hard to eliminate from the carbon nanotube layer resulting in impurities in the carbon nanotube layer. Thirdly, metallic carbon nanotubes are formed in the carbon nanotuble layer which should be but are hard to eliminate. Additionally, the carbon nanotube layer formed by the printing method is inflexible. Accordingly, the TFT is inflexible.
Corresponding reference characters indicate corresponding parts throughout the several views. The exemplifications set out herein illustrate at least one embodiment of the present method for making the thin film transistor, in at least one form, and such exemplifications are not to be construed as limiting the scope of the invention in any manner.